The present invention relates to electronic signal processing for loudspeakers and in particular to extending the low-frequency capability of loudspeakers.
Conventional electromagnetic loudspeaker drive units have two principal limits on their maximum acoustic output capability: excursion of the cone, and heat buildup. Excessive cone excursion adds distortion to the signal creating a desire to limit the cone excursion. Further, the drive unit temperature rises above tolerable limits if the electrical power-handling ability of the voice coil is exceeded and there is insufficient capacity for removing the resulting heat from the coil. Overly high temperatures ultimately result in a failure of the voice coil insulation, wire, and/or bonding of the voice coil to its former as the temperature of the internal parts becomes so great that electrical insulation and glue systems fail.
The maximum acoustic output limits may be changed if the loudspeaker drive unit is enclosed in a sealed or a vented box or a box equipped with a passive radiator in addition to the main driver. The maximum acoustic output limits may be further changed in more complex enclosures containing combinations of sealed sub-enclosures, vented sub-enclosures, or chambers equipped with passive radiators.
The limits on excursion of the loudspeaker drive unit at audio frequencies may also be changed by the presence of the enclosure because the acoustical load on the driver may be changed by the presence of the enclosure. The electrical power-handling ability may be changed by the presence of the enclosure because the enclosure typically adds to the thermal resistance of the system, and thus a given power input will produce a greater voice coil temperature rise for a driver enclosed in a box compared to a driver in free air.
Additionally, complete loudspeaker systems, as opposed to conventional drive units alone, have additional limits imposed on them due to upper limits on velocity of air in ports, or passive radiators undergoing excessive excursion. High velocity of air in the ports may cause extraneous noise, and passive radiator low frequency maximum excursion may be different from the maximum low frequency excursion of the principal drive units.
Good loudspeakers are designed for flat low-frequency response down to a practical lower limiting frequency, typically using methods explicated by Beranek and Locanthi in the 1950's. Beranek and Locanthi proposed electrical analogies for the electrical and mechanical systems of loudspeakers. These electrical analogies were brought to wide use as a practical system of measurements and application of those measurements by Thiele and Small in the 1960's and 70's. Complete low-frequency loudspeaker design work today is strongly influenced by the papers of Thiele and follow-on work by Small. Thiele produced a catalog of low-frequency responses, modeling loudspeakers as electrical high-pass filters. The models showed various alignments varying flatness of response, steepness of roll-off below the cutoff frequency, potential electrical equalization, group delay, excursion vs. frequency, and other factors. The Thiele-Small parameters have become the most prominent metric used nationally and internationally for the exchange of information about drivers, and have had enormous positive economic impact.
Low-frequency loudspeaker design today is typically an act of balancing a variety of specifications affecting bandwidth, frequency response over the bandwidth, maximum level capacity and its variation with frequency, various distortions, and cost. Among the target frequency response curves available for design from sources such as Thiele, some include separate electrical equalization before the power amplifier. Such equalization may be provided by an underdamped high-pass filter, with peaking of the high-pass filter response at the corner frequency of the high-pass filter made a part of the overall design.
An unaided (i.e., receiving an unfiltered input signal) loudspeaker mechanical and acoustical radiation system has a frequency response showing a particular low-frequency rolloff. Accurate sound production (i.e., a flat frequency response) may be extended to a frequency below the rolloff of the unaided loudspeaker mechanical and acoustical radiation system by providing electrical equalization in the form of an underdamped high-pass filter. Such electrical equalization increases the excursion of the associated loudspeaker driver at the peaking frequency of the high-pass filter and at frequencies around the peaking frequency. However, although such electrical equalization has the benefit of extending the system response below the rolloff frequency of the unaided loudspeaker mechanical and acoustical radiation system, because the electrical equalization increases the power below the rolloff frequency, the equalization raises both the electrical power dissipated as heat below the rolloff frequency and the excursion around and at the rolloff frequency, as shown in one example system and Thiele response alignment by Newman. These increases in heat and excursion may exceed a speaker's limits.
Once the utility of extending the bandwidth with a peaking high-pass filter became known, several inventors took the idea a step further to make the high-pass filter dynamic by various means, and with a varying fit to the excursion capability and power limits of the driver. Unfortunately, such attempts have failed to achieve the best possible fit of bandwidth extension while staying within the excursion and thermal limits of drivers.
Further, electrical equalization which includes a boost capability may be used to extend the frequency range downwards, but may also cause a reduction in the maximum sound pressure level capability vs. frequency typically by the same amount as the equalization vs. frequency response curve of the high-pass filter. Thus, a need remains for a system and method for extending low frequency performance of conventional loudspeaker driver-box systems, for example, open back, closed box, vented box, and their more complex variants composed of combinations of these types of parts, having limitation in their low-frequency response range and maximum sound pressure level capability vs. frequency.
The above described material and other related material is discussed in the following publications:    Beranek, Leo L., Acoustics, McGraw-Hill, New York, 1954;    Burg, T. C., Gao, X., Dawson, D. M., “Robust control for the improvement of loudspeaker low-frequency response,” Southeastcon '93 Proceedings, IEEE, 1993;    “Improving Loudspeaker Signal Handling Capability,” Application Note 104, That Corporation, Milford, Mass.;    Locanthi, B. N., “Application of Electric Circuit Analogies to Loudspeaker Design Problems,” IRE Trans. Audio PGA-4 (1952), reprinted J. Audio Eng. Soc., vol. 19, pps 775-785 (1971);    Newman, Raymond J. “Particular vented box loudspeaker system based on a sixth-order Butterworth response function,” J. Acoust. Soc. Am., vol. 55, issue S1, April, 1974, pp. S29-30;    Small, Richard H., “Efficiency of Direct-Radiator Loudspeaker Systems,” J. Audio Eng. Soc., vol. 19, no. 10, 862-863, November 1971;    Small, Richard H., “Direct Radiator Loudspeaker System Analysis,” J. Audio Eng. Soc., vol. 20, no. 5, pp. 383-395;    Small, Richard H., “Vented-Box Loudspeaker Systems—Part 2: Large-Signal Analysis,” J. Audio Eng. Soc., vol. 21, no. 6, pp. 438-444, July/August 1973;    Thiele, A. N., “Loudspeakers in Vented Boxes: Parts I and II,” J. Audio Eng. Soc., vol. 19 no. 5 May, 1971, pp. 382-392 and no. 6 June, 1971, pp. 471-483; a reprint of Proc. IRE (Australia), vol. 22, p. 487-, 1961.